1. Field of the Invention
The present invention relates to an accumulator fuel injector for a Diesel engine and, more particularly, to an accumulator fuel injector wherein an air-fuel mixing performance is enhanced by accumulating the fuel at an especially high pressure in an accumulator into which the fuel is adapted to be delivered from a fuel injection pump under a high pressure and then injecting the fuel from nozzle holes at that especially high pressure.
2. Background of the Prior Art
An accumulator fuel injector typically has a basic construction, for example as shown in FIGS. 1, 31 and 32. In a body 2,202,302 of an accumulator fuel injector 1,201,301 a fuel inlet 3,203,303 is connected in communication with nozzle holes through a valve closing pressure chamber 4,204,304, a check valve 5,205,305, an accumulator 6,206,306 and an injection valve 7,207,307 which is provided with a valve closing spring 9,209,309, a valve closing pressure receiving surface 10,210,310 and a valve opening pressure receiving surface 11,211,311. The injection valve 7,207,307 is pushed, on the one hand, toward the valve closing side by a resultant force comprising a tension force of the valve closing spring 9,209,309 and a pressure in the valve closing pressure chamber 4,204,304 acting on the valve closing pressure receiving surface 10,210,310 and, on the other hand, toward the valve opening side by a pressure within the accumulator 6,206,306 acting on the valve opening pressure receiving surface 11,211,311.
A accumulator fuel injector having such a basic construction functions as follows in order to enhance the fuel-air mixing performance by injecting the fuel at an especially high pressure.
(a) Accumulation Function
First of all, the fuel is sent pressurized into the valve closing pressure chamber 4 through the inlet 3 by a fuel injection pump 16 during its delivery stroke and then into the accumulator 6 through the check valve 5, and is compressed so as to be accumulated therein at an especially high pressure (for example 700-1500 Kg/cm). When the pressure in the valve closing pressure chamber 4 becomes equal to that in the accumulator 6 at the end of the delivery stroke of the pump 16, the check valve 5 is closed by a check valve spring 17.
To this point, since the valve closing force (which acts on the valve closing pressure receiving surface 10) and the valve opening force (which acts on the valve opening pressure receiving surface 11) are almost equal, the injection valve 7 is closed by force provided by the valve closing spring 9.
(b) Injection Starting Function
When the injection pump 16 makes it suction stroke, the pressure in the valve closing pressure chamber 4 decreases abruptly by escaping to the pump 16 and hardly acts on the valve closing pressure receiving surface 10. Thereupon, the valve opening force which acts on the valve opening pressure receiving surface 11 opens the injection valve 7 against the valve closing spring 9, so that the fuel accumulated at a high pressure in the accumulator 6 is injected through the nozzle holes 8 at a high pressure and expands.
(c) Injection Finishing Function
As the pressure in the accumulator 6 decreases as the fuel injection advances, the valve opening force which acts on the valve opening surface 11 also decreases gradually. When the pressure in the accumulator 6 decreases to a valve closing set up pressure Pc, the valve opening force which acts on the valve opening pressure receiving surface 11 gets smaller than the tension force of the valve closing spring 9. At that time, the fuel injection is finished as the injection valve 7 is closed by the valve closing spring 9.
In the accumulator fuel injector having such a typical basic construction, the accumulator 206,306 comprises only one accumulation chamber as shown in FIG. 31 (refer to U.S. Pat. No. 4,436,247) and FIG. 32 (refer to U.S. Pat. No. 4,561,590).
Accordingly, since the fuel injection pressure P varies linearly relative to the fuel injection quantity Q as shown in Diagram X of FIG. 5, the following problems are encountered therewith. That is, (1) the smaller the engine load becomes, or the further the injection advances, the sooner the injection pressure decreases, and (2) the fuel injection is carried out only intermittently during light load or no-load operation.
The reasons for those phenomena will now be explained. Generally, since the liquid fuel such as light oil used for a Diesel engine experiences a the volumetric strain under pressure, the pressure thereof varies proportionally relative to the quantity charged into a certain volume.
As shown in FIG. 5, when the maximum injection quantity Q of the fuel, the maximum injection starting pressure Pmax and the injection finishing pressure Pc (the valve closing set up pressure for the injection valve) are defined respectively, the maximum injection starting point A and the injection finishing point B are determined accordingly.
In the conventional embodiment as abovementioned, the relation between the fuel accumulation quantity and the accumulation pressure in the accumulator 206,306 , namely between the injection quantity Q and the injection pressure P, is indicated as one linear line which connects the injection finishing point B and the maximum injection starting point A at FIG. 5 as shown in Diagram X. Therefore, the less the injection quantity Q becomes, or the further the injection advances, the sooner the injection pressure p decreases linearly when compared with the maximum injection starting pressure Pmax. Thereupon, the combustion performance soon becomes worse because the fuel atomization as well as the fuel spray penetration also deteriorate rapidly.
As known in the art of the injection valve 207, 307, the valve opening set up pressure Po is selected to be higher than the valve closing set up pressure Pc by the pressure corresponding to the differential area between the little smaller valve opening pressure receiving surface and the relatively larger valve closing pressure receiving surface.
While the pressure in the accumulator 206,306 increases from the valve closing set up pressure Pc to the valve opening set up pressure Po, the injection valve 207,307 is not opened and the fuel injection is not performed. In order to increase the pressure therein from the valve closing set up pressure Pc to the valve opening set up pressure Po, a certain charging quantity XQ is required for opening the valve 207,307.
During a no-load or a light load operation, there are some cases where one time charging fuel quantity which is charged from the injection pump 16 to the accumulator 206,306 becomes less than the valve opening charging quantity XQ. In this case, the pressure does not reach the valve opening set up pressure Po by such one time charging quantity, the fuel injection is not performed, and a misinjection is caused. When the pressure reaches the valve opening set up pressure Po after a plurality of chargings, all the fuel quantity charged heretofore is injected at a time.
In that manner, since the fuel injection is performed only intermittently, the angular velocity of the crank shaft varies seriously, the engine revolutions become unsteady, and in certain circumstances, the engine can not keep on running and may stall.